Induced draft heaters are well known and various apparatus has been devised to supply an efficient fuel/air mixture for combustion in such heaters. In addition to promoting fuel efficiency, induction draft heater design objectives also include safety and control of exhaust stack temperatures to permit venting through plastic flue pipe or at or near ground level. For example, U.S. Pat. No. 4,204,832 to Miller discloses a gas burner utilizing an induction fan disposed in the exhaust vent. The fan is controlled thermostatically and is capable of operating at more than one speed. The stated purpose of the thermostatically controlled induction fan is to control the proportion of secondary air entering the heater to control the temperature of the flue gases and the combustion of the fuel gas in the flame 13. As stated in the patent, the primary air and the fuel gas are mixed in a conventional manner.
It is conventional to have an adjustable air aperture at the air inlet proximate to the gas inlet. For example, U.S. Pat. No. 1,955,622 to Dayton shows, inter alia, a control plate that can affect the volume of air entering a conical air mixer to control the air/fuel ratio. U.S. Pat. No. 4,790,268 to Eising discloses an induction type, gas-fired water heater which aspirates air by venturi effect, viz., by a concentric venturi having a large diameter end communicating with the atmosphere and a small diameter end disposed adjacent the outlet of the burner.
The induction fan in an induced draft heater affects both the intake air flow and the exhaust product flow out the flue. Certain patents suggest that a variable speed fan may have utility for various reasons. For example, U.S. Pat. No. 5,524,556, to Rowlette et al. discloses an induced draft gas furnace having an induced draft fan with a variable speed to compensate for changes in back pressure in the vent line. The patent recognizes that the induction fan affects the fuel/air mixture by controlling the air flow into the combustion process. A selected constant flow of air is provided by controlling the speed of the fan motor and the volume of air moved by the induction fan is determined by measuring the fan parameters proportional to motor torque and speed which are read on a continual basis and processed by a microprocessor. The microprocessor responds to input data by controlling the pulse width output and the drive duty cycle for the fan motor.
U.S. Pat. Nos. 4,334,855, 4,340,355 and 4,533,315 to Nelson each disclose a gas fired heater having a two-speed induction blower with high and low speeds, corresponding to a high and low firing rate. In the Nelson patents, the means for accomplishing the adjustment of the fuel/air ratio is a valve which varies the gas supply in response to a pressure differential on either side of a constriction in the exhaust vent.
U.S. Pat. No. 5,112,217 to Ripka et al. discloses a gas fired heater with a constant fuel flow rate and a variable air flow rate established by a variable speed induction fan. The induction fan speed is determined by sensing upon and digitizing the radiation generated by the burner which is compared by a computer to a reference measure of radiation. The computer adjusts the fan speed to increase or decrease the air flow such that the sensed radiation comes to approximate the desired standard radiation. The radiation sensed is described in the specification as in the upper ultra violet, visible or near infrared ranges and is transmitted from the interior of the burner to the exterior by means of a fiber optic cable. A radiation source is provided, e.g., a light emitting diode, which serves as the standard to which the flame radiation is compared. Accordingly, the burner includes a calibration methodology and apparatus that accommodates the changed operating parameters associated with sensor age and the degradation of the sensor.
Notwithstanding the various apparatus that have been proposed in the field, there remains a need for an induced draft heater having an air/fuel mixture control that is both efficient and simple. Accordingly, it is therefore an object of the present invention to provide an air/fuel mixture control having those qualities.